U.S. Pat. No. 3,755,477 describes a process for producing fluorinated aliphatic hydrocarbons which comprises fluorinating a halogenated aliphatic hydrocarbon, including trichloroethylene, by reaction in the gas phase with hydrogen fluoride in the presence of a steam-treated and calcined chromium oxide catalyst prepared by a multi-step process. Example 22, col. 5, with trichloroethylene as a raw material, shows formation of CF.sub.3 CH.sub.2 Cl at 6.2/1 HF/C.sub.2 HCl.sub.3 mol ratio, 3.6 seconds contact time and 290.degree. C. reaction temperature. In this example, the trifluorinated product is reported as the sole product evidently formed to the exclusion of the desired CClF.sub.2 CH.sub.2 Cl product.
U.S. Pat. No. 3,258,500 describes a process for the catalytic vapor phase reaction of HF with various halohydrocarbons, including trichloroethylene, at 150.degree. to 700.degree. C. reaction temperatures in the presence of a catalyst that consists essentially of a heat-activated anhydrous chromium (III) oxide which may be supported on alumina. This catalyst is highly active. Although trichloroethylene is disclosed among the halogenated aliphatic hydrocarbons that may be employed, there is no example of its use as a raw material in the disclosed process. Examples 17 through 20, however, show use of other halo and polyhaloethylenes at 300.degree. to 400.degree. C.. In each case, the main reaction product is that wherein the halo groups other than fluoro originally present in the halo or polyhaloethylene have all been replaced by fluoro groups.
GB No. 1,000,485 describes a process for the preparation of organic fluorinated compounds by fluorination of halo-olefins in the gaseous phase and at a temperature preferably within the range 200.degree. to 400.degree. C.. The catalyst consists essentially of partially fluorinated alumina impregnated with one or more polyvalent metal halides. The polyvalent metal may be chromium, cobalt, nickel, or manganese. The total content of polyvalent metal halide expressed as oxide is not more than 15% by weight of the partially fluorinated (70-80%) alumina expressed as alumina. Example 1 (Table 1) shows reaction of trichloroethylene with HF over such catalyst yields CF.sub.3 CH.sub.2 Cl as the major product with CClF.sub.2 CH.sub.2 Cl as a very minor product at 280.degree. to 360.degree. C. In addition, the patent states that if fluorination of the catalyst is excessive, the activity of the catalyst is impaired (page 3, column 2, lines 85-87).
The references do not show how to produce selectively both 1,1-difluoro-1,2-dichloroethane and 1-fluoro-1,1,2-trichloroethane while minimizing the production of 1,1,1-trifluorochloroethane.
The process of the instant invention achieves the desired high degree of selectivity by minimizing the formation of the trifluorochloroethane, through catalyst selection and control of the reaction variables as discussed below and illustrated in the Examples.